1. Field of the Invention
This invention relates to an optical amplifier suitable for wavelength division multiplexing (WDM) and an optical fiber which can be applied to the optical amplifier.
2. Description of the Related Art
In recent years, a production technique and a use technique of an optical fiber of a low loss (for example, 0.2 dB/km) have been established, and an optical communication system wherein an optical fiber is used for a transmission line has been put into practical use. Further, in order to compensate for the loss of an optical fiber to allow long-haul transmission, use of an optical amplifier for amplifying an optical signal has been proposed and such optical amplifier has been put into practical use.
As a technique for increasing the transmission capacity of one optical fiber, wavelength division multiplexing (WDM) is available. In a system to which WDM is applied, a plurality of optical signals having different wavelengths from each other are wavelength division multiplexed by an optical multiplexer, and resulting WDM signal light is sent out into an optical fiber transmission line. On the reception side, the received WDM signal light is demultiplexed into individual optical signals by an optical demultiplexer, and transmission data are reproduced based on the optical signals.
As one of conventional optical amplifiers, an optical amplifier is known which includes an optical fiber (doped fiber) doped with a rare earth element and a pump light source for pumping the doped fiber so that the doped fiber may have a gain band which includes the wavelength of an optical signal. For example, as an optical amplifier for amplifying an optical signal of, for example, a wavelength 1.55 .mu.m band, an EDFA (erbium-doped fiber amplifier) has been developed which includes an erbium-doped fiber (EDF) and a laser diode which outputs pump light of a wavelength 0.98 .mu.m band or 1.48 .mu.m band.
In order to incorporate an EDFA into a system to which WDM is applied, it is preferable that output control can be performed for optical signals of individual channels. In an ordinary EDF, since erbium as a dopant is doped uniformly in a core, a gain characteristic (wavelength dependency of the gain) is determined uniquely, and accordingly, it is impossible to effect output control of optical signals of individual channels independently of each other.
Taking the foregoing into consideration, the inventor of the present invention has published a structure and a use technique of a specific EDF for effecting output control of optical signals of individual channels in TECHNICAL DIGEST of OPTICAL AMPLIFIERS AND THEIR APPLICATIONS (OAA) (July, 1996).
The technique is described briefly with reference to FIGS. 1 and 2. Referring first to FIG. 1, reference numeral 2 denotes a cross sectional construction of the EDF in the proximity of a core. The EDF shown has a first region 4 made of Si--Ge glass and a second region 6 made of Si--Al glass. Each of the first and second regions 4 and 6 is doped with erbium (Er). The first region 4 is positioned in the proximity of the center of the core while the second region 6 is provided in such a ring-shaped configuration that it surrounds the first region.
In FIG. 1, reference numerals 8 and 10 denote distributions of the power density in a diametrical direction when pump light of a wavelength 0.98 .mu.m band and pump light of another wavelength 1.48 .mu.m band are propagated in an EDF, respectively. The mode field diameter of the pump light of the wavelength 0.98 .mu.m band is comparatively small as denoted by reference numeral 12 while the mode field diameter of the pump light of the wavelength 1.48 .mu.m band is comparatively large as denoted by reference numeral 14. Accordingly, with this EDF, erbium doped in the first region 4 can be selectively pumped by pump light of the wavelength 0.98 .mu.m band while erbium doped in the second region 6 can be selectively pumped by pump light of the wavelength 1.48 .mu.m band.
Referring now to FIG. 2, there are shown two gain characteristics obtained with the EDF of FIG. 1. In FIG. 2, the axis of ordinate indicates the normalized value of the emission (or emission cross section), and the axis of abscissa indicates the wavelength (nm). It is known that the wavelength characteristic of the emission corresponds to the wavelength characteristic of the gain (dB/m) of the EDF per unit length, that is, the gain characteristic. Based on the fact that the first and second regions 4 and 6 have different glass compositions, where the EDF is pumped with pump light of the wavelength 0.98 .mu.m band, such a comparatively steep gain characteristic as denoted by reference numeral 16 is obtained, but where the EDF is pumped with pump light of the wavelength 1.48 .mu.m band, such a comparatively moderate gain characteristic as denoted by reference numeral 18 is obtained. Accordingly, if an optical amplifier is formed from an EDF of the type described above and at least two pump light sources and is applied to a WDM system, then the output powers of optical signals of individual channels can be controlled independently of each other.
An optical amplifier which employs the EDF described above with reference to FIG. 1 may possibly suffer from two drawbacks based on the following reasons.
(1) Since the second region 6 is provided at a position where the power density of pump light of the wavelength 1.48 .mu.m band is comparatively low, the gain of the optical amplifier may not possibly be made sufficiently high.
(2) Although the first and second regions 4 and 6 are pumped principally with pump light of the 0.98 .mu.m band and pump light of the 1.48 .mu.m band, respectively, since the two pump lights are different in mode field from each other, the first region 4 is pumped also with pump light of the 1.48 .mu.m band while the second region 6 is pumped also with pump light of the 0.98 .mu.m band. As a result, where the EDF is applied to WDM, there is the possibility that the dynamic range of each input power may not be made large.
Here, the "dynamic range of each input power" signifies an allowable range, where WDM signal light including optical signals of first and second channels are supplied to the optical amplifier, of the input power of one of the optical signals within which the requirement that the powers of the amplified optical signals of the first and second channels be equal to each other when the input power of the other of the optical signals of the first and second channels is fixed is satisfied.